1. Field of Invention
The present invention relates to cement bond logging and more particularly to methods and apparatus for measuring the attenuation rate of sonic energy traversing casing cemented in a borehole.
2. The Prior Art
In a well completion, a string of casing or pipe is set in a borehole and cement is forced into the annulus between the casing and the borehole primarily to separate oil and gas producing formations from each other and from water bearing strata. Obviously, if the cementing fails to provide a separation of one zone from another, then fluids under pressure from one zone may be able to migrate and contaminate an otherwise productive nearby zone. Migration of water, in particular, produces undesirable water cutting of a producing zone and possibly can make a well non-commercial.
It is a problem to obtain an accurate picture of conditions behind a casing because of the difficulty of propagating signals through the casing wall. Various prior proposals to determine the separation effectiveness, (i.e., the blocking or sealing characteristics) of the cement behind the casing have not been entirely successful in clearly determining the effective presence of cement in the annulus between the casing and the formation. Further, it has not been possible to measure reliably the quality of the cement bond between the casing and the cement.
The mere presence or absence of cement in the annulus between the casing and formation is valuable information, however, this does not provide a complete picture of the cement conditions. While cement may be present in the annulus, channels or inadequate sealing may still permit fluid communication between adjacent formations.
Use of the term "bond" in connection with the relationship of cement to the casing or the formation is somewhat vague, sinnce adherence along the entire boundary between the casing and the cement or between the cement and formation is not necessary to prevent fluid communication between adjacent porous zones. All that is necessary of a bond is that the relationship prevents the migration of fluids. Hereafter, reference to bond will mean that separation of zones by cement is adequate to prevent fluid migration between the zones.
Several prior developments for obtaining a measure of the quality of a cement bond relative to the casing have been disclosed in U.S. Pat. Nos. 3,291,274, 3,291,248 and 3,292,246. These systems generally utilize acoustic principles where an acoustic signal is transmitted between a transmitter and a receiver. The amplitude of the early arrival signal (this early arrival usually is the casing signal since the acoustic energy under average conditions generally travels faster in the casing than in the surrounding cement or formation) at the receiver is measured as a determination of the quality of the bond of cement to the casing. If a good bond existed, the casing signal would be expected to be attenuated because of the energy dissipated from the casing to the cement and surrounding formations, whereas if no bond or a poor bond existed the casing signal would be relatively unattenuated.
A more refined technque for determining the quality of cement in the annulus between the casing and the formations is disclosed in U.S. Pat. No. 3,401,773 entitled, "Method and Apparatus for Cement Logging of Cased Boreholes" by Judson D. Synnott, III and assigned to the assignee of the present invention. In this technique the amplitude of a reverberated early (casing) signal arrival is recorded and additionally, the total energy of a selected later portion of the sonic signal is obtained by integration to provide a second indication of the quality of the cement bond. Even in the absence of a weak casing arrival, the additional step of observing the total energy obtained by integrating a later portion of the signal in this manner can confirm the presence of cement in the casing-annulus-formation system. Details of related mathods may also be had by reference to U.S. Pat. No. 3,401,772 entitled, "Methods for Logging Cased Boreholes" by Frank P. Kokesh, which is assigned to the assignee of the present invention.
While the foregoing methods and apparatus provide very useful information, it is desirable to more precisely determine the quality of the cement bond. It has been established that the energy content of the acoustic logging signals arriving at the receiver depends on other factors than the quality of the cement bond to the casing or the integrity of the cement column (sometimes called cement quality). The following factors were found to have substantial effect on signal arrivals: receiver sensitivity; the formation hardness; eccentering of the acoustic logging tool; the high temperature environment and the temperature variations in the well bore; type of casing; and the diamater of the borehole and casing as well as their shape or geometry.
It will be appreciated, therefore, that it is highly desirable to provide methods and apparatus for determining the quality of the cement bond in a cased borehole, which methods and apparatus reduce the detrimental effects of the aforementioned factors.